Pacific ENSO Update

1st Quarter 1999 - Vol. 5 No. 1

CURRENT CONDITIONS

SST

SSTs point toward drier than normal winter and early spring rainfall
conditions in near-equatorial regions east of 160° East.

By July 1998, the equatorial sea surface temperatures (SSTs) in the
central Pacific indicated that weak La Niña (cold episode)
conditions had developed. The rapidness of the cooling suggested that a
strong La Niña event could develop. To date equatorial SSTs have
continued to fall much more slowly, and in January 1999 the temperatures
had become 1.5-2°C (2.7-3.6°F) cooler than normal between 120°
West and 170° West. This indicates a moderate La Niña event
and agrees with our predictions in the last two Newsletters. The
equatorial cold tongue associated with the La Niña now extends from
the west coast of South America westward to about 160° East. Some
climate models are predicting La Niña conditions to peak in early
1999, while others suggest that the La Niña will not peak until the
August-October 1999 timeframe. Some models also predict that cooler than
normal equatorial SSTs will continue into 2000, while others suggest that
SSTs will return to near normal values in late 1999. We expect to see a
peak around February-March of 1999 and a return to near normal conditions
by the last quarter of the year. The colder than normal equatorial SSTs
should maintain stronger than normal easterly low level equatorial winds
and drier than normal atmospheric conditions along the equatorial belt
east of 160° East. However, conditions become wet only 200 miles
north and south of the dry equatorial zone.

SOI

The SOI points to near normal to above normal rainfall for most of
Micronesia, but less than normal rainfall will prevail during winter and
early spring in the Marshall Islands and in near-equatorial regions.
Tropical cyclone activity will return to normal as the La Niña
weakens and allows tropical cyclone development to occur in central and
eastern parts of Micronesia.

The Southern Oscillation Index (SOI) began to rise and became positive
in May of 1998. It has hovered between +1 and + 1.5 since July 1988, but
it jumped to near +2.0 in January 1999. It will likely not rise much more,
and will begin to return to more normal values by September. This behavior
of the SOI supports moderate La Niña conditions through the
Northern Hemisphere spring. The higher than normal SOI indicates higher
than normal pressures over Tahiti and continued lower than normal
pressures over northern Australia, reflecting the very active Australian
monsoon. The monsoon has, in fact, been very active from the South China
Sea to northwestern Australia and across the southwest Pacific to the
Solamon Islands and Vanauatu. Episodes of monsoon surges have extended to
Fiji, bringing some periods of heavy rains to the parched island nation.
La Niña conditions have shifted the upward branch of the Walker
Circulation (the primary east-west tropical circulation pattern) to the
west, bringing heavy rains to Indonesia, the southern South China Sea, the
southern Philippines, western Micronesia, and northern Australia. While
good rains have returned to the Samoa region, the region will likely see
slightly below normal rainfall as the South Pacific Convergence Zone
(SPCZ) is frequently anchored to the west by the strong Australian
monsoon.

SEA LEVEL HEIGHTS

Sea level heights should rise slightly in the western Pacific as water
temperatures warm to deeper levels and increase the volume of basin water.
Sea levels should be lower than normal in the central and eastern
equatorial Pacific where La Niña waters are colder than normal, the
thermocline is shallow, and water volume is reduced. Sea level heights
should return to near normal values throughout the Pacific basin by early
2000. These heights can be temporarily and locally modulated by stronger
than normal trade winds from the east, from monsoon surges from the west,
and from tropical cyclones.

TROPICAL CYCLONE ACTIVITY

Tropical cyclone activity is expected to be heavy from northwest
Australia and across northern Australia eastward into the Southwest
Pacific. Significant activity is expected to remain west of the
international date line. A rare February tropical storm (TS Iris) spun up
in central Micronesia, as westerly winds from the Australian monsoon
spread northward across the equator. Since 1970, there have been two
tropical storms, two typhoons, and one tropical depression in the western
North Pacific during February.

For nearly four days as Iris tracked westward from south of Guam to the
north of Palau, cloud clusters associated with the tropical cyclone and
cloud bands in its outer circulation contributed the major part of
near-record February rains across the Mariana Islands.

Northern-Southern Hemisphere twin cyclones could form in April or May,
affecting central and western Micronesia and northern Australia. By
summer, the Northern Hemisphere monsoon is expected to become active,
bringing episodes of southwesterly wind and heavier than normal rains to
most of Micronesia. In 1999, tropical cyclone activity should return to
normal in terms of numbers and locations of development.

Mahalo Nui Loa to the Environmental Verification and
Analysis Center

On behalf Chip Guard of the University of Guam, Tom Schroeder, Cheryl
Anderson, and Ray Tanabe of the University of Hawaii and Jerry Norris of
the Pacific Basin Development Council, I would like to thank Mark
Morrissey, Andrew Wood, Brad McGavock, and their staff at the
Environmental Verification and Analysis Center (EVAC) for serving as
editor of the Pacific ENSO Update. Mark assumed the role as guest editor
when Alan Hilton, the former editor of the update, reported in April 1998
for an extended assignment as ship captain for the NOAA Corps.

The EVAC folks edited and produced three issues of the newsletter and
did an outstanding job. Production of the Pacific ENSO Update occurred in
addition to their center project, Schools of the Pacific Rainfall Climate
Experiment (SPaRCE), which educates students in climate information
throughout the Pacific. Details of EVAC's ongoing projects appeared in the
September 1998 newsletter of the Pacific ENSO Update. EVAC's letter SPaRCE
can be read at their website < http://gulfstream.ou.edu >. We
sincerely thank all of them for their assistance. The Pacific ENSO
Applications Center hopes to continue to work cooperatively with EVAC in
furthering education and information dissemination on climate variability.

Ray Tanabe has taken over as editor as of this issue and we congratulate
and thank him for his expanded role in the Pacific ENSO Application
Center.

Mahalo for all the effort supporting a better understanding of ENSO
in the Pacific region!

A Short Assessment of the Pacific ENSO Applications
Center Rainfall Predictions For Micronesia and American Samoa During the
1997-98 Drought

Figure 1 compares the predicted, the
observed, and the normal rainfall values in inches at the major
Micronesian Islands and at Pago Pago, American Samoa during the 1997-1998
El Niño-induced drought. Figure 2
shows the predicted and observed rainfall in terms of percent of normal
rainfall. Overall, for the 12-month period, the predictions were very
accurate. However, at certain locations, there were some problems in
predicting the onset of the drought and in anticipating the return of
rains that broke the drought. For example, the onset of the drought was
predicted a few weeks too early for Guam, Saipan, Palau, and Yap. And, the
drought onset was predicted to start a few weeks too late for Kosrae and
American Samoa. It was even more difficult to predict the reestablisment
of the rains. For example, the drought was forecast to end several weeks
too early for Guam, Saipan, and American Samoa, and several weeks too late
for Yap, Palau, and the Marshall Islands. Part of the problem in
determining the end of the drought is that there is no real definition of
the end. Defining the end of the drought is an
area that requires a great deal of work ince different locations will
likely have different criteria for the definition. A forthcoming WERI
Technical Report will break down the 1997-1998 rainfall predictions in
more detail.

LOCAL VARIABILITY SUMMARIES:

Hawaii:
State: The trade winds, strong at times, marked the first ten days of the month across the
Hawaiian Islands. These trades brought occasional periods of heavy showers over the windward
side of the island of Hawaii, but no damage was reported. Rainfall totals for the rest of the state
were relatively modest.

The initial period of trade wind weather was followed by the passage of a weak cold front across
the island chain on 11 and 12 February. This system was accompanied by rainfall totals of less
than 0.50 inches at most locations across the state and a few reports of 1.00 to 1.50 inches from
the windward side of the island of Hawaii. The trailing high pressure ridge settled in just to the
north of the state bringing stable weather conditions with land and sea breezes through 18
February.

On 19 February, a weak shear line pushed across the state and lined itself up with the island of
Hawaii the following day. An upper level shortwave trough then dropped in from the northwest
and became superposed over the shear line. This combination resulted in an unstable airmass
over the eastern end of the island chain and helped trigger heavy showers and thunderstorms on
the morning of 20 February. Urban and small stream flood advisories were subsequently issued
for the eastern districts of the island of Hawaii. Showers and thunderstorms continued to develop
through the day, which prompted the issuance of a flash flood watch for the windward districts of
the island. The watch was later expanded to cover the entire island of Hawaii and all of Maui.
The axis of the upper level trough moved east of the island chain during the early morning hours
of 22 February and the subsiding air to the west of the trough stabilized the airmass. As much as
13 inches of rain fell over the windward side of the island of Hawaii during the three-day event
leaving some low-lying areas flooded, though no major damage was reported.

Minimal rainfall was observed statewide through 25 February where light to moderate trade
winds were accompanied by stable conditions aloft. A weak shear line stalled over the island of
Kauai on 26 February and dropped up to 1.50 inches on the windward slopes of the island. The
rest of the state remained dry through the end of the month.

American Samoa: Although rains returned to the islands after several
months of severe drought conditions, they were less than expected. We are
anticipating a continued increase in rainfall, but values will likely be
slightly drier than normal for the Samoa region.

In the last Pacific
ENSO Update, we anticipated rainfall amounts to reach near normal or
slightly above normal values from November through February as the South
Pacific Convergence Zone (SPCZ) became anchored over or near the islands.
While the SPCZ has increased rainfall in the region, the very strong
Australian monsoon has frequently anchored the SPCZ west of the islands.
This has reduced rainfall amounts below the levels predicted in the last
Newsletter for November through February. In fact, November was the
hottest November ever recorded at Pago Pago. Pago Pago rainfall values for
October were 4.30 inches (64%) and for November were 3.22 (30%). Amounts
increased in December to 10.46 inches (72%) and in January to 11.36 inches
(90%), still well-below the values we anticipated in our last Newsletter.
The moderate La Niña event will likely keep the remainder of the
rainy season and the upcoming dry season somewhat drier than normal.
However, we expect the following rainy season to be near normal. Rainfall
predictions from the Climate Prediction Center support this line of
thinking as well. As anticipated, tropical cyclone activity in the
Southern Hemisphere moved west of the international date line, reducing
the threat to the Samoa region. Early season activity was somewhat closer
to the date line than expected, affecting Fiji.

Guam/CNMI: With an active monsoon expected across the Northwest
Pacific, we expect Guam and the CNMI to be wetter than normal in the
summer of 1999.

The last quarter of 1998 was significantly drier than
expected, especially in the CNMI. In October, November, and December,
rainfall at Guam International Airport (GIA) was 9.27 (77%), 6.79 inches
(83%), and 3.94 inches (73%) respectively, while that at AAFB was 8.02
inches (62%), 6.37 inches (70%), and 4.18 inches (70%). At Saipan
International Airport (SIA), the respective values for October, November,
and December were 4.68 inches (43%), 2.95 inches (51%), and 1.21 inches
(31%). Capitol Hill was wetter with 6.52 inches (54%), 5.24 (72%), and
2.86 inches (60%), respectively. Tinian rainfall for the 3-month period
was wetter than SIA but likely drier than Capitol Hill. The NASA rain gage
network located at the Rota Resort and Country Club on Rota and the Rota
Airport showed October amounts of around 5.7 inches (44%), 4.8 inches
(53%) and 2.8 inches (47%). The Sabana area was probably a little wetter
than the Airport and Golf Resort areas, but not significantly so.

Rainfall picked up in the first quarter of 1999 throughout the region,
especially in February when a rare tropical cyclone dropped more than 10
inches of rain in 3-4 days. January rainfall was highly variable on Guam,
with 4.70 inches (106%) at GIA and 3.46 inches (61%) at Andersen AFB. Some
tropical cyclone activity in April or May should render the spring wetter
than normal. We expect a very active monsoon across the western Pacific
from July through September, and this should bring significantly wetter
than normal conditions to the Mariana Islands. Rainfall will become
slightly wetter than normal to normal in the last quarter of 1999.

Tropical cyclone activity could affect the Mariana Islands as early as
March, but it is more likely in April or May. The greatest threat of
tropical cyclone activity will occur from September to mid-December.

Our rainfall predictions through March 2000 for Guam/Rota and
Saipan/Tinian are:

Federated States of Micronesia:
We anticipate somewhat wetter than normal conditions for most of the
FSM. Near-equatorial regions east of 160° East will be drier than
normal through the winter and early spring of 1999.

Rainfall over most
of the FSM was above normal for the period October 1998-January 1999. This
was largely the result of westward-propagating tropical disturbances
developing in the trade wind convergence zone near 5-8° North. In
southwestern portions of the FSM, heavy rains occasionally spread
northward from the active Australian monsoon that extended from the South
China Sea, southeastward across northern Australia and into the Solamon
Islands. A portion of the western FSM, extending from Woleai through
Polowat, was about 15-20% drier than normal during the period. This
dryness extended northward into Yap and Ulithi during November and
December. Easterly equatorial winds brought dry conditions to Nauru and
equatorial areas east of 160°East.

Yap State: October, November, and December rainfall at the Yap
Airport was 16.11 inches (135%), 7.95 inches (88%), and 17.65 inches
(85%). Over the 3-month period, both Ulithi Atoll and Woleai Atoll had
about 20-25% less rainfall than Yap Island. In January, rainfall rose to
near normal amounts at Yap (7.65 inches--104%). Woleai and Ulithi had
somewhat below normal amounts with 10.04 inches (94%) and 5.97 inches
(90%), respectively. During February, rainfall increased over the western
FSM as the periphery of Tropical Storm Iris dropped several inches of rain
on Yap. Rainfall is expected to be above average through spring. By
summer, rainfall will become near normal to slightly below normal through
the summer as the normal monsoon activity moves well to the north. During
fall and winter, rainfall is expected to approach more normal amounts. Yap
State islands could be subjected to tropical storms and typhoons in April
and May, with a larger threat from September until mid-December.

Chuuk State: October, November, and December rainfall at Weno
Island 12.62 inches (94%), 15.02 inches (145%), and 13.83 inches (128%),
respectively. At Lukunoch, October-December rainfall averaged about 12.7
inches (114%). These amounts were considerably higher than anticipated in
our last predictions. Conditions were significantly drier at Polowat where
October-December rainfall averaged 6.33 inches (64%), but this amount was
less than we predicted. In January, Weno Island saw 13.79 inches of rain
(129%), Lukunoch observerd 16.83 inches (158%), and Polowat measured 7.71
(96%). These were above the values anticipated in our last Newsletter. The
Chuuk Islands are expected to see near to above normal rainfall through
spring. By summer, rainfall will then become slightly below normal through
the summer as the normal monsoon activity moves well to the north. During
fall and winter, rainfall is expected to approach more normal amounts.
Although tropical cyclone activity was much reduced in 1998, Chuuk State
islands could be subjected to a tropical storm or typhoon in April or May
and again from October-mid December.

Pohnpei State: October, November, and December rainfall values
were14.81 inches (89%), 17.35 inches (110%), and 19.91 (131%). For
Pingalap, rainfall averaged 14 inches (88%) over the period, and for
Nukuoro rainfall averaged 11 inches (93%). Most of Pohnpei State's
rainfall came tropical disturbances that developed in the trade wind
convergence zone that meandered between 4° North and 8° North.
Nukuoro rainfall was higher than expected as the strong Australian monsoon
occasionally replaced the equatorial easterlies with moist northwesterly
winds and intermittant heavy rains. In fact, Kapingamarangi likely
received some relief from the monsoonal rains. In January, Pohnpei had
18.22 inches of rain (139%), Pingelap measured 18.33 inches (136%), and
Nukuoro was drenched with 36.63 inches (312%). Wetter than normal
conditions are expected to continue until late spring in all but
near-equatorial locations. By summer, rainfall will then become slightly
below normal through the summer as the normal monsoon activity moves well
to the north. During fall and winter, rainfall is expected to approach
more normal amounts.

Kosrae State: October (7.75 inches -- 48%) and November (8.36 
53%) rainfall at the Kosrae Airport was dry as expected due to increased
equatorial easterly winds resulting from the current La Niña event.
Rainfall amounts at Tafunsak (57%) and Utwa (58%) were slightly greater
during the period. However, rainfall amounts increased earlier than
expected, as tropical disturbances developed in the trade wind convergence
zone closer to the equator than expected. December amounts at the Airport
increased to 23.41 inches (161%) and January amounts were even higher at
29.76 inches (207%). At Tafunsak and Utwa, December values were 18.88
inches (130%) and 24.54 inches (169%), respectively, and January values
were 26.56 inches (185%) and 32.16 inches (223%), respectively. Rainfall
is expected to remain above normal through the spring, but will then
become below normal through the summer as the normal monsoon activity
moves well to the north. During fall and winter, rainfall is expected to
approach more normal amounts. Kosrae has a low risk of tropical cyclone
activity, especially in non-El Niño years.

Palau: Palau can expect much above normal rainfall until summer and
then above normal rainfall for the remainder of the year.

Rainfall at
Koror in October, November and December was 19.05 inches (137%), 17.43
inches (154%), and 13.57 inches (113%), respectively. At Peleliu,
October-December rainfall averaged 15.46 inches (120%). January 1999
rainfall at Koror and Peleliu was even greater at 24.83 inches (232%) and
24.69 inches (231%), respectively. The northern atolls (Kayangel, etc.)
were slightly drier than Koror, and the southern atolls (Tobi, etc.) were
likely similar to Koror. The heavy rainfall for Palau was the result of
the combination of westward moving tropical disturbances that developed in
the trade wind convergence zone and the active Australian monsoon that
occasionally spread northward into the southern Palau region. Tropical
Storm Iris passed north of Palau in February 1999, keeping regional
rainfall way above normal. Rainfall is expected to remain above or near
normal through spring. Then rainfall will become slightly below normal
through the summer as the normal monsoon activity moves well to the north.
During fall and winter, rainfall is expected to approach more normal
amounts. Palau could experience a tropical cyclone in April or May and
again from October-December 1999.

Marshall Islands: Despite a relatively wet summer and fall in the
Marshall Islands, the development of a moderate La Niña is expected
to reduce winter and early spring rains. At Majuro(representative
of southern atolls), rainfall for October, November, and December was
14.45 inches (104%), 13.57 (106%), and 11.48 (97%), respectively. At
Kwajalein (representative of northern atolls) rainfall for the last three
months of the year was 14.69 inches (123%), 7.95 inches (75%), and 5.85
inches (72%). In January, Majuro and Kwajalein had rainfall amounts of
7.23 inches (86%) and 4.55 (100%). Enhanced easterly winds associated with
the moderate La Niña is expected to create drier than normal
conditions in the Marshalls until about April-May 1999. The increased
monsoon activity expected across the western North Pacific during summer
is not likely to increase rainfall in the Marshall Islands.

Tropical cyclone activity could be a threat to the Marshall Islands in
October and November 1999.

We anticipate the following rainfall amounts for the Marshall Islands:

ACKNOWLEDGEMENTS and FURTHER INFORMATION

The information contained in the LOCAL VARIABILITY SUMMARIES section
and elsewhere in this issue of the Pacific ENSO Update has been
drawn from many sources. Further information may be obtained by contacting
your local National Weather Service office, or the individuals and
institutions listed below:

NOAA National Weather Service - National Centers for Environmental
Prediction (NCEP) - CLIMATE PREDICTION CENTER (CPC):World Weather
Building, Washington D.C. 20233.Contact CPC at 301-763-8155 for more
information on the ENSO Advisory, the Long-Lead Outlook for the Hawaiian
Islands, and other publications discussed in this bulletin.

University of Guam (UOG) WATER AND ENERGY RESEARCH INSTITUTE (WERI):
Lower campus, University of Guam UOG Station, Mangilao, Guam
96923 Contact C. Guard or M. Lander at (671)735-2685 for more info on
tropical cyclones and climate in the Pacific Islands.

University of Hawaii (UH) School of Ocean and Earth Science and
Technology (SOEST) DEPARTMENT OF METEOROLOGY: HIG #331, 2525
Correa Road, Honolulu, Hawaii 96822 Contact Dr. T. Schroeder at
808-956-7476 for more information on hurricanes and climate in Hawaii.

Publication of the
Pacific ENSO Update is funded in part by Grant Number
NA46GP0410 from the National Oceanic and Atmospheric Administration
(NOAA) Office of Global Programs. The views expressed herein are
those of the author(s) and do not necessarily reflect the views of
NOAA or any of its sub-agencies.